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76                                        FLORA  INDICA-
being expanded and made lighter, it immediately ascends, its
place being supplied by air from colder regions. Tims, since
no two places have the same temperature, and since the tem-
perature constantly changes, even in the same place, the at-
mosphere is kept in constant motion.
As the amount of aqueous vapour which is capable of re-
maining suspended in the atmosphere is directly proportional
to the temperature, ascending currents of air finally become
so cooled that condensation or precipitation takes place; arid
the nearer to saturation the air is before it begins to ascend,
the sooner it will reach a sufficiently low temperature for
condensation. We can therefore understand why mountain-
chains (which impede the direct course of the currents, and
force them to ascend) cause precipitation of the moisture of
an atmosphere which has already traversed, without any con-
densation, a great extent of level country,
The direction of the wind is primarily dependent upon the
sun's position, and is a very complex phenomenon, in conse-
quence of the perfect fluidity of the air. On the open sea, at
a sufficient distance from land to escape its influences, the
trade-winds, owing to the iutcrtropical 'heat, blow with groat
regularity towards the equator, or rather towards a point im-
mediately under the sun's-position, varying therefore with the
season of the year. Their direction is not due north and
south, but more or less towards the west. This is hi conse-
quence of their retaining the momentum proper to the lati-
tude whence they start, in their advance towards the equa-
tor, where the motion of a point OH the earth's surface (duo
to its revolution round its own axis) is a maximum. They
therefore lag behind, as it were, and appear to blow from
the north-east iu the northern hemisphere, and front the
south-east in the southern hemisphere, The4 presence of
land interferes with the regularity of the tradtMviiulx; and
where it occurs iu large manses, it hc< tomes so mueli more
heated than the ocean, that it attracts the aerial current* to-
ward* itself, mid hi'iicc completely changes the direction of
the whuL